{  Statement: http://acmp.ru/?main=task&id_task=529
   Verdict: Accepted
}
uses
    SysUtils, Math;

var
    a,b,c,d:longint;

begin
reset(input,'input.txt');
rewrite(output,'output.txt');
read(a,b,c,d);
write(sqrt((c-a)*(c-a)+(d-b)*(d-b)):0:16);
end.
